Circadian Clock Proteins LHY and CCA1 Regulate SVP Protein Accumulation to Control Flowering in Arabidopsis

Abstract

The floral regulators GIGANTEA (GI), CONSTANS (CO), and FLOWERING LOCUS T (FT) play key roles in the photoperiodic flowering responses of the long-day plant Arabidopsis thaliana. The GI-CO-FT pathway is highly conserved in plants. Here, we demonstrate that the circadian clock proteins LATE ELONGATED HYPOCOTYL (LHY) and CIRCADIAN CLOCK-ASSOCIATED1 (CCA1) not only repressed the floral transition under short-day and long-day conditions but also accelerated flowering when the plants were grown under continuous light (LL). LHY and CCA1 accelerated flowering in LL by promoting FT expression through a genetic pathway that appears to be independent of the canonical photoperiodic pathway involving GI and CO proteins. A genetic screen revealed that the late-flowering phenotype of the lhy;cca1 double mutant under LL was suppressed through mutations in SHORT VEGETATIVE PHASE (SVP), a MADS box transcription factor. Yeast two-hybrid analysis demonstrated an interaction between SVP and FLOWERING LOCUS C, and genetic analysis indicated that these two proteins act as partially redundant repressors of flowering time. SVP protein accumulated in lhy;cca1 plants under LL. We propose a model in which LHY and CCA1 accelerate flowering in part by reducing the abundance of SVP and thereby antagonizing its capacity to repress FT expression under LL.

Isolation of suppressors of late flowering and abnormal flower shape phenotypes caused by overexpression of the SHORT VEGETATIVE PHASE gene in Arabidopsis thaliana

“…The 35S:SVP plants produced more rosette leaves before bolting than WT plants under LD and LL conditions (Fujiwara et al 2008). Flower shape of the 35S:SVP is also affected by overexpression of the SVP gene (Brill and Watson 2004;Masiero et al 2004;Liu et al 2007).…”

“…The first report on isolation of an svp mutant demonstrated that it had an early-flowering phenotype without displaying other distinguishing features (Hartmann et al 2000), and thus the svp mutant seemed to pass more rapidly through the vegetative development stage. Recently we found that double loss-of-function of two clock genes, LHY and CCA1, caused late flowering under LL (Fujiwara et al 2008). We have also shown that svp and flc mutations suppress the late-flowering phenotype under LL (Fujiwara et al 2008).…”

“…The lhy;cca1 mutation delayed flowering time of Arabidopsis under continuous light (LL), but accelerated flowering under light/dark cycles such as LD and SD (Mizoguchi et al 2002. Our recent genetic studies indicated that two mutations, svp and flc, partially suppress the late-flowering phenotype of lhy;cca1 (Fujiwara et al 2008). Based on these results, we proposed that both an internal biological clock and external rhythms are required for proper development of Arabidopsis.…”

“…Transgenic Arabidopsis overexpressing the SVP gene (35S:SVP) has been described previously (Fujiwara et al 2008). Seeds were imbibed and cold treated at 4°C for 3 days in the dark before germination under light conditions.…”

“…To isolate novel genes that functionally interact with SVP to control flowering time and flower shape, EMS mutagenesis was performed on 35S:SVP (Fujiwara et al 2008) seeds, and M 2 populations were screened for suppressors of late flowering and abnormal flower shape phenotypes of 35S:SVP ( Figure 1). In total 12,000 M 2 seedlings were screened for individuals that flowered earlier than 35S:SVP plants and had normal flower shapes similar to WT (Ler) plants under LL.…”